Field of the Invention
The invention relates to a wash oil for use as an antifouling agent in gas compressors and the use of such a wash oil.
Description of Related Art
Cracked gas compression systems are multi-stage systems and comprise multiple gas compressors provided with interstage coolers and afterstage coolers at the compression discharge. The coolers are typically heat exchangers that remove the heat of the compression from the gas feed and reduces its temperature to approximately the temperature at the compression intake. Another use of coolers is the reduction of the actual volume of gas flowing to the high pressure cylinders while the separator after the intercooler is installed to remove the condensed liquid.
Cracked gas compression systems such as in ethylene plants are prone to fouling. Foulants may be found deposited in the compressor, aftercoolers or both, in particular on the compressor's casings, bearings, blades, seals, rotors and discharge lines. Other locations areas of fouling may include interstage cooler shells and tubes, cooling water sides and knockout drum plates and trays (Global Journal of Pure and Applied Science, Volume 11, 2005, pages 99 to 105).
Fouling of the cracked gas compressor system is mostly caused by polymerization and condensation reactions involving materials present in the cracked gas that polymerize and deposit on the internal surfaces of the compressor and aftercoolers. Such polymeric fouling affects the cracked gas compression system in a number of ways, such as reducing the compressor's efficiency by increasing the energy consumption and by causing compressor vibrations which may lead to reduction in throughput and run length. Furthermore, fouling deposits found in the interstage cooler tubes and shells reduce heat transfer by raising the inlet temperature of the next stage. Also, pressure drop across the cooler may increase as well by reducing the inlet pressure and efficiency of the next stage.
As mentioned, fouling comprises polymerization and condensation deposits which result from the reaction of compounds such as butadiene and styrene or other unsaturated compounds present in the cracked gas. It is being suggested that the reactions primarily responsible for fouling are free radical polymerization and diels-alder condensation reactions.
The radical polymerization reaction is caused by heat and enhanced by the presence of peroxides (see scheme 1).

Diels-alder condensation reactions also contribute to the problem which results in the formation of heavy material that condenses on the inner surfaces of the compressor and gradually dehydrogenates. Such condensation products are potential source of hard coke-like material that can damage seals and other parts of the internals of the compressor (see scheme 2).

In the past, several methods have been applied to control the process of gas compressor fouling in the ethylene industry. The commonly applied methods for reducing or inhibiting fouling include the use of appropriate coatings, wash oil, water injections, anti-foulants and other design and operating considerations.
Compressor coatings are used to reduce corrosion and foulant deposition in process gas compressors and are typically applied to the diaphragms and rotor assemblies during maintenance downtime. By providing such coatings the surface characteristics of the compressor are changed such that an adhesion of the polymer to the surface is prevented.
Another approach is the addition of so-called anti-foulants which reduce the impact of fouling in various ways. Anti-foulants are chemical species to prevent reactions or terminate polymer chain formation. In particular, inhibitors are used to reduce the free radical polymerization rates and metal deactivators can be applied to prevent catalysis of hydro peroxide decomposition. It is also possible to add dispersants as anti-foulants to reduce polymer deposition.
Another common approach for inhibiting fouling of cracked gas compressors is the addition of water in order to lower the gas discharge temperature and the gas volume. Water vaporizes in the compressor stage and by doing so it absorbs heat of the compression. The decrease in temperature reduces the fouling rates and is a key component of fouling control. The obvious drawback when adding water to the compressor is the potential for corrosion and erosion.
An even further and often applied strategy for reducing fouling is to dissolve the polymer deposits after its formation. This can be done by adding a solvent (or also called wash oil) that is capable of removing the deposit and is added directly to the compressor. The basic properties of a suitable wash oil are a high aromatic content and a high boiling point. Suitable wash oils should be furthermore free of fouling precursors and suspended solids.
The aromatic content of a promising wash oil is in the range of 60 wt % and higher, preferably above 80 wt %. The higher the aromatic content of a wash oil the higher its potential to dissolve the polymer deposits.
Wash oils with a high boiling point will ensure that the wash oil remains liquid allowing it to dissolve and remove polymer from the metal surfaces and minimize the deposition of solids. Initial boiling points of greater than 200° C. are recommended.
Furthermore, the wash oil should be low in monomer content and free of polymer and solids itself in order not to add to the fouling problem. While high in aromatic content, the wash oil should be essentially free of styrene and diene compounds. Since the wash oil may at least partially evaporate in the compressor, it should thus also be free or almost free of any suspended solid.
There are many different wash oils on the market, though C9+material typically available as a recycle from the gasoline hydrotreator (GHU) it is preferably used in naphtha cracking plants. Said material has low diene content and the styrene content is typically about 0.3 wt % or less. The C9+stream contains 60 to 80% aromatics and has a boiling end point of about 230 to 260° C.
Other wash oils offered by manufacturers are pyrolysis gasoline derivatives or naphthalene depleted fractions of aromatic streams from oil refineries.
However, the presently available wash oils are of a rather high price adding to the overall costs of the gas cracking process.
It was therefore an object of the present invention to provide a wash oil for use as an anti-fouling agent in gas compressors which combines the requirements for suitable wash oil at a reasonable price.